How can I apply a stylesheet to only a container frame and not the widgets within it in Qt designer?

Question

I am currently designing a GUI in Qt Designer, and in a certain part of it, I want 4 QPushButtons in a QGridLayout within a frame widget. I want to apply a black border to this QFrame called menu_frame.

However, if I apply it, every button within it also gets a black border, breaking my design idea completely.

How can I only apply a black border to the frame widget itself, not to anything within it?

The stylesheet I use is:

border: 1px solid black;

Note: I did not create any widget in my code, every widget was dragged to my GUI in Qt Designer.